In a constant velocity universal joint, which is used to construct a power transmission system for automobiles and various industrial machines, two shafts on a driving side and a driven side are coupled to each other to allow torque transmission therebetween, and rotational torque can be transmitted at a constant velocity even when each of the two shafts forms an operating angle. The constant velocity universal joint is roughly classified into a fixed type constant velocity universal joint that allows only angular displacement, and a plunging type constant velocity universal joint that allows both the angular displacement and axial displacement. In a drive shaft configured to transmit power from an engine of an automobile to a driving wheel, for example, the plunging type constant velocity universal joint is used on a differential side (inboard side), and the fixed type constant velocity universal joint is used on a driving wheel side (outboard side).
Irrespective of the fixed type and the plunging type, the constant velocity universal joint includes, as main components, an inner joint member, an outer joint member, and torque transmission members. The outer joint member includes a cup section and a shaft section. The cup section has track grooves formed in an inner peripheral surface thereof and configured to allow the torque transmission members to roll thereon. The shaft section extends from a bottom of the cup section in an axial direction. In many cases, the outer joint member is constructed by integrally forming the cup section and the shaft section by subjecting a rod-like solid blank, that is, a round bar to plastic working such as forging and ironing or processing such as cutting work, heat treatment, and grinding (see FIG. 4 and FIG. 8 of Patent Literature 1).
In Patent Literature 1, there is disclosed steps of manufacturing an outer joint member, which integrally includes a cup member and a shaft member, through forging. The forging is described with reference to FIG. 26a to FIG. 26e. First, a rod-like blank, that is, a billet 170 (FIG. 26a) is subjected to forward extrusion molding so that a preform 172 (FIG. 26b), which includes a shaft section 174 and a solid main body section 176, is obtained. Next, the solid main body section 176 is subjected to upsetting so that an intermediate preform 178 (FIG. 26c), which is constructed by the shaft section 174 and a solid large-diameter section 180, is obtained. Next, the solid main body section 178 is subjected to backward extrusion and formed into a cup section 184, which is literally opened at one end, so that a preforged product 182 (FIG. 26d) is obtained. After that, the cup section 184 of the preforged product 182 is subjected to ironing so that an ironed product 186 (FIG. 26e) that is finished with high accuracy is obtained. In the course of the backward extrusion and ironing, on an inner side of the cup section, there are formed track grooves 188 serving as rolling surfaces for torque transmission balls in the case of a ball type constant velocity universal joint, and there are formed track grooves (not shown) in the case of a tripod type constant velocity universal joint.
The shaft section 174 molded by the forward extrusion (FIG. 26b) is retained by a lower portion molding die (not shown) throughout all of subsequent steps of upsetting (FIG. 26c), backward extrusion (FIG. 26d), and ironing (FIG. 26e). Further, the upsetting is performed in a single step (FIG. 26c).
Incidentally, an outer joint member (long stem type) including a shaft section longer than a standard may sometimes be used. For example, in order to equalize lengths of a right part and a left part of the drive shaft, the long stem type is used for a constant velocity universal joint on the inboard side that corresponds to one side of the drive shaft. In this case, the shaft section is rotatably supported by a support bearing. Although varied depending on vehicle types, the length of the shaft section of the long stem type is approximately from 300 mm to 400 mm in general. The outer joint member of the long stem type has a long shaft section, and hence there is a difficulty in integrally forming the cup section and the shaft section with high accuracy. Therefore, there is known an outer joint member in which the cup section and the shaft section are formed as separate members, and both the members are joined through friction press-contact (Patent Literature 2).
An overview of the friction press-contact technology for the outer joint member described in Patent Literature 2 is described below. First, as illustrated in FIG. 27, a cup member 72 and a shaft member 73 are joined through the friction press-contact to form an intermediate product 71′. Next, burrs 75 on a radially outer side of a joining portion 74 are removed, and hence an outer joint member 71 as illustrated in FIG. 28 is obtained. The burrs 75 are generated on the joining portion 74 of the intermediate product 71′ along with the press-contact. The burrs 75 on the radially outer side of the joining portion 74 are removed through processing such as turning. Accordingly, a support bearing (rolling bearing 6: see FIG. 1) to a shaft section of the outer joint member 71.
Although illustration is omitted, the intermediate product 71′ is processed into a finished product of the outer joint member 71 through machining of a spline, snap ring grooves, and the like, and through heat treatment, grinding, and the like. Therefore, the outer joint member 71 and the intermediate product 71′ have slight differences in shape. However, illustration of the slight differences in shape is omitted in FIG. 27 and FIG. 28 to simplify the description, and the outer joint member 71 being the finished product and the intermediate product 71′ are denoted by the reference symbols at the same parts. The same applies to the description below.